A controversial experiment which poured iron into the Southern Ocean has also poured cold water on the idea that such ‘ocean fertilization’ can mitigate against climate change.
The Lohafex project was investigating suggestions that carbon dioxide can be removed from the atmosphere by promoting algal blooms with iron. Despite protests from some groups, researchers aboard the Polarstern research vessel carried out their experiment this month.
However, the Alfred-Wegener institute, which was backing Lohafex, says “only a modest amount of carbon sank out of the surface layer by the end of the experiment. Hence, the transfer of CO2 from the atmosphere to the ocean to compensate the deficit caused by the LOHAFEX bloom was minor compared to earlier ocean iron fertilization experiments.”
Although the iron did initially stimulate plankton production, predation from small copepods prevented further growth. In addition, previous experiments have led to increases in diatom algae, which form silica shells that sink after the algal blooms, trapping the carbon.
The institute says previous, natural blooms had extracted all the silicic acid, preventing diatom growth. It adds:
Hence a major finding was that other algal groups, although stimulated by iron fertilization, are unable to make blooms equivalent to those of diatoms. Since the silicic acid content in the northern half of the Southern Ocean is low, iron fertilization in this vast region will not result in removal of significant amounts of CO2 from the atmosphere.
Kenneth Coale, director of Moss Landing Marine Laboratories in California, was more upbeat.
He told the BBC, “To date we’ve conducted experiments in what amounts to 0.04% of the ocean’s surface. All have indicated that iron is the key factor controlling phytoplankton growth, and most have indicated that there is carbon flux (towards the sea floor) – this is one that didn’t.”
Image top: Polarstern in the open sea. / Alfred-Wegener-Institut
Image lower: copepod / G. Mazzochi, SZN / Alfred Wegener Institute